As described in Japanese Patent Laying-Open No. 2013-080613 (patent document 1) and Japanese Patent Laying-Open No. 2002-246061 (patent document 2), as one large-capacity storage battery, a redox flow battery (hereafter also referred to as an “RF battery”) is known. Referred to as applications of the redox flow battery are load leveling, as well as momentary drop compensation and backup power supply, and smoothing an output of natural energy such as solar power generation, wind power generation and the like whose massive introduction is prompted.
An RF battery is a battery which performs charging and discharging using as a positive electrode electrolyte and a negative electrode electrolyte an electrolyte containing a metal ion (an active material) having a valence varying by oxidation-reduction. FIG. 7 shows a principle of an operation of a vanadium-based RF battery 300 which uses as a positive electrode electrolyte and a negative electrode electrolyte a vanadium electrolyte containing a V ion. In FIG. 7 a solid line arrow and a broken line arrow in a battery cell 100 indicate a charging reaction and a discharging reaction, respectively.
RF battery 300 includes cell 100 separated into a positive electrode cell 102 and a negative electrode cell 103 by an ion exchange membrane 101 which permeates hydrogen ions. Positive electrode cell 102 has a positive electrode 104 incorporated therein, and a tank 106 provided for the positive electrode electrolyte and storing the positive electrode electrolyte is connected via conduits 108, 110 to positive electrode cell 102. Similarly, negative electrode cell 103 has a negative electrode 105 incorporated therein, and a tank 107 provided for the negative electrode electrolyte and storing the negative electrode electrolyte is connected via conduits 109, 111 to negative electrode cell 103. And by pumps 112, 113, the electrolyte stored in each tank 106, 107 is circulated and thus passed through cell 100 (positive electrode cell 102 and negative electrode cell 103) to perform charging and discharging.
In RF battery 300, normally, a configuration including a cell stack having a plurality of cells 100 stacked in layers is utilized. FIG. 8 is a schematic configuration diagram of a cell stack. A cell stack 10S is formed such that it is composed of a cell frame 20 including a frame body 22 and a bipolar plate 21 integrated therewith, positive electrode 104, ion exchange membrane 101, and negative electrode 105, each stacked in a plurality of layers, and this stack is sandwiched and thus clamped by two end plates 250s. 
In cell stack 10S, positive electrode 104 is disposed at one surface side of bipolar plate 21 and negative electrode 105 is disposed at the other surface side of bipolar plate 21, and a single cell will be formed between adjacent cell frames 20. In cell stack 10S, an electrolyte is passed by a manifold 200 provided to penetrate frame body 22, and a slit 210 formed on a surface of frame body 22 between manifold 200 and bipolar plate 21. In cell stack 10S illustrated in FIG. 8, the positive electrode electrolyte is supplied from a liquid supply manifold 201 via a slit 211 that is formed on one surface side (corresponding to the front side of the sheet of the drawing) of frame body 22 to bipolar plate 21 on the side of positive electrode 104, and the positive electrode electrolyte is drained via a slit 213 that is formed at an upper portion of frame body 22 to a liquid drainage manifold 203. Similarly, the negative electrode electrolyte is supplied from a liquid supply manifold 202 via a slit 212 that is formed on the other surface side (corresponding to the back side of the sheet of the drawing) of frame body 22 to bipolar plate 21 on the side of negative electrode 105, and the negative electrode electrolyte is drained via a slit 214 that is formed at an upper portion of frame body 22 to a liquid drainage manifold 204. Furthermore, at a portion of frame body 22 where slits 211-214 are formed, a protective plate 30 made of plastic and protecting an ion exchange membrane 101 is disposed. Each protective plate 30 has a throughhole formed at a position corresponding to each manifold 201-204 and has a size to cover each slit 211-214. Slit 211-214 covered with protective plate 30 do not contact ion exchange membrane 101, and the ion exchange membrane can be prevented from being damaged by the irregularity of the slits.